Understanding CNC Machining Tolerances: What Engineers Need to Know
Introduction
When designing a machined part, tolerances are one of the most important yet misunderstood elements of engineering. The right tolerance ensures proper fit, smooth function, and long-term reliability—but overly tight tolerances can dramatically increase machining time and cost.
In this guide, we break down what CNC machining tolerances are, how they affect manufacturing, and how to choose the correct tolerance for your application.
What Are CNC Tolerances?
A tolerance defines how much variation is allowed from a specified dimension. Since no machining process can produce a perfect measurement every time, tolerances ensure that parts still fit and function correctly even with small deviations.
Example:
If a dimension is listed as 50.00 mm ±0.05, the acceptable measurement range is 49.95 mm to 50.05 mm.
Why Tolerances Matter
Tolerances directly impact:
Fit: How parts assemble or slide together
Function: Whether components operate as intended
Quality: Precision and reliability in real-world performance
Cost: Tighter tolerances require more time, tools, and inspections
Correct tolerances create balance:
Precise enough to work, but not so tight that manufacturing becomes expensive.
Types of CNC Machining Tolerances
1. Standard Tolerances
Most machinists work within standard tolerances unless the drawing specifies otherwise.
For CNC milling and turning, typical tolerance ranges are:
±0.005 in (±0.127 mm) for general features
±0.001–0.002 in for tighter functional features
These tolerances are cost-effective and suitable for most applications.
2. Geometric Dimensioning & Tolerancing (GD&T)
GD&T provides a symbolic language to describe how parts should be oriented, located, or form-fit relative to each other.
Common GD&T controls include:
Flatness
Perpendicularity
True position
Concentricity
Parallelism
GD&T is essential in aerospace, medical, and automotive applications where precision is critical.
3. Unilateral & Bilateral Tolerances
Bilateral: Example ±0.05 mm (variation allowed on both sides)
Unilateral: Example +0.00/−0.05 mm (variation allowed in only one direction)
Use unilateral tolerances when the part must not exceed a maximum boundary (e.g., clearance fit).
How Tight Tolerances Affect Cost
Every decimal point matters in machining.
Tight tolerances increase cost because they require:
Slower cutting speeds
More precise tooling
Additional setup time
More frequent inspections
Possible use of custom fixturing
General rule:
Only tighten tolerance when function requires it.
When to Specify Tight Tolerances
Use strict tolerances for:
Mating components
Bearing fits
Shafts and bores
Sealing surfaces
Aerospace and medical parts
Precision mechanical assemblies
Avoid tight tolerances on non-critical surfaces or cosmetic features.
How to Choose the Right Tolerance
Consider the following:
1. Purpose of the feature
Does the feature support motion? Alignment? Structural load?
2. Material being machined
Aluminum machines more consistently
Titanium and stainless steel may require looser tolerances due to tool pressure and heat
3. Production volume
Prototypes can hold tighter tolerances more easily than mass production.
4. Assembly method
Interference fits, slip fits, and press fits all require different tolerance bands.
Work With Your Machinist Early
Every project is unique. The best way to determine appropriate tolerances is to collaborate early with your machining partner.
At Universal Machining & Engineering, our team reviews drawings, identifies cost-saving opportunities, and ensures every tolerance is achievable while meeting functional requirements.
Conclusion
Understanding CNC tolerances is key to creating functional, reliable, and cost-effective parts. By applying tolerances strategically, you can keep machining efficient while maintaining the precision your design demands.
Need Help Optimizing Tolerances for Your Next Machined Part?
Our engineering team can review your drawings and recommend the ideal tolerances for performance and cost.
